JP3742662B2 - Magnet suitable for open magnetic resonance imaging - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates generally to magnetic resonance imaging magnets, and more particularly to an open or non-closed magnet that combines a cooled superconducting coil with a ferromagnetic pole piece.
[0002]
[Prior art]
Magnetic resonance imaging (MRI) is now a widely accepted diagnostic procedure and its use is becoming increasingly common. MRI systems require a uniform magnetic field and radio frequency (RF) radiation to create MR images. Various types of magnets are currently used to create a magnetic field. Whatever type of magnet is used, a portion of the magnetic field must be very uniform. This portion of the magnetic field, referred to herein as the imaging volume, is the portion of the magnetic field that encompasses the imaging object. In order to obtain a good quality image, the uniformity of the magnetic field within the imaging volume is required. Therefore, it would be advantageous to have a magnet that can create a relatively large imaging volume for many imaging applications.
[0003]
Typically, magnets used for whole body imaging are configured to be housed in a cylindrical structure with a relatively narrow central lumen. The narrow lumen forms an enclosed chamber into which the patient must enter for imaging. These cylindrical configurations provide a number of positive features, but can be difficult in some cases. For example, a significant percentage of patients are sensitive to the closed nature of conventional MRI systems. For these patients, the MRI process is uncomfortable or even unbearable. In addition, some patients are simply too large to fit into the narrow chambers of conventional MRI systems. Difficulties can also arise when a patient is placed in an enclosed chamber connected to an IV system or other medical device. Such closed conventional systems also have difficulties in veterinary applications. Many animals are scared of closed chambers.
[0004]
Various open magnet configurations have been proposed as an accessible alternative to the conventional closed system. One such magnet configuration is described in U.S. Pat. No. 4,875,485. In this configuration, two superconducting coils are arranged in parallel at a distance, and a working space is formed between them. A relatively open space is suitable for accepting patients and is less susceptible to claustrophobia. In this configuration, it is described that a spherical imaging volume with a uniformity of 50 ppm to 100 ppm and a diameter of 20 cm is formed.
[0005]
U.S. Pat. No. 4,924,198 shows another open magnet configuration. In this configuration, the two magnet assemblies are arranged in parallel at an interval. In one embodiment, each magnet assembly includes a superconducting coil and a built-in resistor coil arranged substantially concentrically and concentrically therewith. In this example, in the case of a center magnetic field of 0.5 Tesla, a uniform imaging peak value of 30 ppm and a spherical imaging volume of 20 cm are obtained. In the second embodiment, each magnet assembly includes a pair of superconducting coils. In this example, for 0.5 Tesla, the uniformity is 13 ppm and a spherical imaging volume of 25 cm is obtained.
[0006]
Japanese Patent Application No. 4-141148 shows an open-type magnet configuration with two parallel, spaced superconducting coils. The device also includes a pivot mechanism that allows the coil pair to rotate between a horizontal position and a vertical position. For field strengths of about 0.3 Tesla to 0.5 Tesla, this configuration yields a 30 cm spherical imaging volume with a uniformity of about 30 ppm.
[0007]
In the open magnet configuration described above, an imaging volume with a diameter in the range of 20 cm to 30 cm is formed. While these configurations are effective, it is desirable to provide an open MRI apparatus that can form a larger diameter imaging volume while maintaining a sufficient level of uniformity.
[0008]
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to allow all patients, including those sensitive to closed spaces, to easily enter and exit whole body MR imaging.
More specifically, one object of the present invention is to provide an open magnet for MR imaging that maintains a very uniform magnetic field within a large imaging volume.
[0009]
Another object of the present invention is to facilitate veterinary MR imaging.
It is a further object of the present invention to provide a method for designing a magnet that meets the above objectives.
In the present invention, the above-mentioned object and other objects are achieved by providing an open-type magnet having two magnet assemblies arranged in parallel and spaced apart to form a work space. Each assembly includes an annular superconducting coil and a ferromagnetic field enhancer surrounded by the superconducting coil. The magnet assembly is attached to a C-shaped support frame. The C-shaped support frame is rotatably attached to the pedestal member. Thus, the frame can be rotated between a vertical imaging position and a horizontal imaging position. A field enhancer is a ferromagnetic pole piece specially shaped to improve uniformity within the imaging volume with a non-linear optimized design method.
[0010]
This design method includes the steps of determining the initial shape of the pole pieces, selecting design parameters that affect the magnetic field, and a function of the selected parameters (objectives) that is the difference between the actual magnetic field and a completely uniform magnetic field. And determining the value of the parameter that determines the optimum shape of the pole piece by minimizing the objective function. The initial shape of the pole piece is preferably a substantially cylindrical shape, with the main parameter being the perturbation point on the surface of the generally cylindrical end face. Minimization is performed using known minimization techniques that automatically adjust the parameters until the minimum of the objective function is reached. In performing the minimization step, the objective function is evaluated using the finite element method.
[0011]
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.
The subject matter considered as the invention is pointed out with particularity in the claims. However, the invention can be best understood by referring to the following description in conjunction with the accompanying drawings.
[0012]
【detailed explanation】
Throughout the drawings, the same reference symbols represent the same elements. 1-4 shows an MR open magnet apparatus 10 of the present invention. The open magnet device 10 includes two magnet assemblies 12 that are spaced apart to form a work space. Each assembly 12 includes an annular superconducting coil assembly 14 and a magnetic field enhancer 16. Each magnetic field enhancer 16 is a substantially cylindrical ferromagnetic pole piece surrounded by one annular coil assembly 14. Each superconducting coil assembly 14 is attached to a corresponding magnetic field enhancer 16 by a bracket (not shown). The two magnet assemblies 12 are arranged such that the end faces of the magnetic field enhancer 16 are on a plane parallel to each other, and their centers are on a line extending perpendicular to the plane.
[0013]
The two magnet assemblies 12 are supported by a support frame 18 made of a non-magnetic material such as stainless steel or aluminum. The frame 18 is C-shaped in order to support the magnet assemblies 12 so as to face each other with a space therebetween. By positioning the magnet assembly in this manner, a sufficiently large space is formed between them and a patient can be received for MR imaging. A magnetic field is formed in the space between them by the magnet assembly 12. The support frame 18 is rotatably attached to the pedestal 20 via a pivot mechanism 22. The pivot mechanism 22 allows the support frame 18 and the magnet assembly 12 to rotate between two imaging positions. At one imaging position shown in FIGS. 1-3, the parallel end faces of the magnetic field enhancer 16 are in the horizontal direction, and the patient can be imaged in a lying posture. FIG. 4 shows another imaging position. At this position, since the support frame is rotated by 90 ° about the axis of the pivot mechanism 22, the parallel end face of the magnetic field enhancer 16 is in the vertical direction. In this position, the patient can be imaged while standing. A suitable pivot mechanism for this purpose is described in the aforementioned Japanese Patent Application No. 4-141148.
[0014]
Superconducting coil assembly 14 may be any type of annular coil of superconducting material. For example, a suitable superconducting coil assembly is described in the aforementioned US Pat. No. 4,924,198. U.S. Pat. No. 4,924,198 describes an assembly comprising an epoxy impregnated superconducting coil supported by an aluminum ring. The coil and ring are surrounded by a heat shield and the entire assembly is enclosed in a vacuum enclosure. Cooling is performed with a two-stage low-temperature cooler. Since superconducting coil assembly 14 does not constitute an inventive aspect of the invention by itself, it is not necessary to describe superconducting coil assembly 14 in more detail for a complete understanding of the invention.
[0015]
When operating alone, the magnetic field generated by the coil assembly 14 has relatively low uniformity and low magnetic field strength. The portion of the magnetic field having uniformity suitable for imaging, ie the imaging volume, is small. By including a ferromagnetic field enhancer or pole piece 16, the present invention increases the magnetic field strength and greatly improves the uniformity of the magnetic field, so that a spherical imaging volume 24 with a good uniformity and a large diameter is formed. It is formed. That is, by properly defining the shape of the pole pieces, the magnetic field can be changed to be more uniform within the larger spherical imaging volume 24.
[0016]
Ideally, the pole piece 16 is shaped to maximize uniformity within the spherical imaging volume 24 while keeping the weight and volume of the pole piece 16 to a minimum. The ideal pole piece shape of the present invention is determined using a non-linear optimization design method. This nonlinear optimization design method achieves the desired magnetic field strength and uniformity by automatically adjusting selected magnet design parameters via computer simulation. In general, this design method involves the determination of a function of design parameters (referred to as an objective function) that measures the deviation between the actual magnetic field and a perfectly uniform ideal magnetic field. Once determined, a design is then found that produces a magnetic field that most accurately reflects a perfectly uniform magnetic field by then minimizing the objective function.
[0017]
As described above, the first step in the design method is to determine an objective function that is the difference between the actual magnetic field and a completely uniform magnetic field. Since it is a function of the selected parameter, the objective function changes as the parameter changes. When creating the objective function, the evaluation point of the objective function must also be determined. An evaluation point refers to a particular point on the surface, primarily on the surface, within which the magnetic field strength and uniformity must be determined for evaluation. The next step is to define an initial model of the pole piece. This is essentially the first shape, which must be reshaped by the design method to a better magnetic field. The initial model of the pole piece is usually determined empirically. In the case of the present invention, a substantially cylindrical shape is selected.
[0018]
Once the initial shape is defined, the design parameters to be changed must be selected and the design parameter constraints must be defined. Design parameters are numerical variables related to magnet design. The main parameter is the geometry of the pole pieces, but other physical quantities such as superconducting coil current, winding and relative position can also be taken into account. The most critical parameter for reducing the magnetic field inhomogeneity near the iron pole piece is the shape of each end face facing the imaging volume, referred to herein as the pole face. The pole faces are represented mathematically by a number of points on the surface of each pole face, which can be perturbed up or down in a vertical direction parallel to the longitudinal axis of the pole piece. These points are therefore called perturbation points.
[0019]
The selected parameters can only be changed within physically reasonable limits. Therefore, certain constraints must be imposed on the parameters. For example, the perturbation point on the pole face cannot be adjusted too close to the imaging volume. This is because a predetermined gap must be maintained from the imaging volume. Another constraint on the perturbation point is that the vertical distance between two adjacent perturbation points cannot exceed a given value. If this is exceeded, it becomes impossible to manufacture the pole face surface. Considering superconducting coils, other constraints must be added. For example, the inner diameter of a superconducting coil cannot be smaller than the diameter of a pole piece, and the amount of current that can pass through a superconducting material can be limited.
[0020]
The final step is to create a pole piece shape that provides a magnetic field that most accurately reflects a perfectly uniform magnetic field by minimizing the objective function. Minimization is performed using a non-linear optimization algorithm. Since the objective function is very non-linear, an algorithm is needed to converge to the minimum of the objective function that is robust and can incorporate specific constraints. Computer Journal, 1965, Vol. 7, pp. 308-313, cited in the paper by J. Nerda and Earl Meade, “Simplex Method for Function Minimization” (“A Simplex Method for Function Minimization”). , "Computer Journal, Vol. 7, 1965, pages 308-313, by JA Nelder and R. Mead), which satisfies these requirements and is a preferred method. The above Nelda and Meade method converges the objective function to a minimum value without the need to evaluate the partial derivative of the objective function with respect to the design parameters. This is an important consideration. This is because the objective function cannot be used without numerical analysis.
[0021]
The basic idea of the Neruda and Meade method is to start with an arbitrary simplex in the effective space and converge through a series of transformations of the simplex to the minimum of the objective function. The transformation is to compare the function value at each vertex of the simplex and replace the highest vertex with another point. The new point is determined by a series of mathematical operations. A more detailed explanation is given in the above paper.
[0022]
As described above, in the Nelda and Meade method, the value of the objective function must be compared at each vertex of the simplex. To do this, you must know the value of the objective function. In the present invention, using a well-known finite element method, a magnet model is created by a computer, and an objective function is calculated at each vertex. Each time the parameters are adjusted during the process, a new finite element mesh is created and a finite element solver is run to calculate a new value of the objective function.
[0023]
FIGS. 5-7 show three different examples of shaped ferromagnetic pole pieces determined by the method described above. As a starting shape for the pole pieces, a substantially cylindrical shape was chosen empirically. Given this shape, the shape of the end face facing the imaging volume, referred to herein as the pole face, becomes a critical design parameter for reducing the magnetic field inhomogeneity near the ferromagnetic pole piece. Thus, in the embodiment of FIGS. 5-7, the optimization method described above is used to provide the most appropriate pole face using other design parameters, such as the height and diameter of the generally cylindrical shape. The shape was determined.
[0024]
Specifically, FIG. 5 is a cross-sectional view of a first embodiment of a shaped magnetic field enhancer or pole piece 116. The pole piece 116 includes a first cylindrical portion 118, a second cylindrical portion 120 having a larger diameter than the first cylindrical portion, and a frustoconical portion 122 that joins the first cylindrical portion and the second cylindrical portion. The superconducting coil assembly 14 is attached to a first cylindrical portion 118 that is longer than the second cylindrical portion 120. Thus, the pole piece 116 is a generally cylindrical member having two end faces 124, 126. The first or outer end face 124 adjacent to the first cylindrical portion 118 faces the opposite side of the spherical imaging volume 20, while the second end face 126 or pole face is the spherical imaging volume 20. Looking towards A shallow magnetic field enhancing recess 128 is formed in the magnetic pole surface 126. The particular shape of the recess 128 determined by the above design method is such that the entire combination produces a magnetic field with a large uniform imaging volume.
[0025]
The first embodiment of the pole piece of FIG. 5 provides excellent uniformity, but is considerably heavier. Another embodiment of a shaped pole piece designed to reduce weight is shown in FIGS. Specifically, FIG. 6 shows a cross-sectional view of a second embodiment of a molded pole piece 216. The second embodiment includes a first cylindrical portion 218, a second cylindrical portion 220 having a larger diameter than the first cylindrical portion, and a truncated cone portion 222 that joins the first cylindrical portion and the second cylindrical portion. This is the same as the first embodiment of FIG. The pole piece 216 differs from the first embodiment in that a conical recess 230 is formed on the outer end face 224. The conical recess 230 serves to significantly reduce the total weight of the pole pieces 216. The formation of a magnetic field enhancing recess 228 in the pole face 226 results in an optimal level of uniformity within a relatively large imaging volume. The shape of the recess 228 is determined according to a design optimization method that automatically adapts to the presence of the conical recess 230. Accordingly, the magnetic field enhancing recess 228 is slightly different from the corresponding magnetic field enhancing recess 128 of the first embodiment 116.
[0026]
When used in the open magnet configuration of the present invention, the pole piece 216 of FIG. 6 is preferably made of iron and has a pole face diameter of 1.4 meters. The pair of pole pieces weighs about 21000 pounds. The superconducting coil assemblies 14 each have 300,000 ampere turns and the central magnetic field strength is 0.35 Tesla. The calculated uniformity for such a configuration is 12 ppm within a spherical imaging volume with a diameter of 30 cm and 69 ppm within a spherical imaging volume with a diameter of 40 cm.
[0027]
FIG. 7 is a cross-sectional view of a third embodiment of the molded pole piece 316. Similar to the first and second embodiments, the molded pole piece 316 includes a first cylindrical portion 318, a second cylindrical portion 320 having a larger diameter than the first cylindrical portion, and the first And a truncated cone portion 322 joining the cylindrical portion and the second cylindrical portion. In the embodiment of FIG. 7, weight reduction is achieved by providing a plurality of concentric annular grooves 332 in the outer end surface 324. Again, the formation of the magnetic field enhancement recess 328 in the pole face 326 results in an optimal level of uniformity in a relatively large imaging volume. The shape of the recess 328 is determined according to a design optimization method that automatically adapts to the presence of the annular groove 332. Accordingly, the recess 328 is slightly different from the corresponding magnetic field enhanced recess in the first and second embodiments.
[0028]
The pair of pole pieces 316 of FIG. 7 made of iron and having a pole face diameter of 1.4 meters weighs about 12,100 pounds. The calculated uniformity for an open magnet with a pair of pole pieces 316 and operating at a magnetic field strength of 0.31 Tesla is 25 ppm within a spherical imaging volume with a diameter of 25 cm and a spherical imaging with a diameter of 30 cm. It is 40 ppm in the volume.
[0029]
Thus, an open magnet for MR imaging has been described that allows most patients to easily enter and exit and produces a large, uniform imaging volume. A useful method for designing the optimum shape of the pole pieces in the MR magnet is also disclosed. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications can be made thereto without departing from the spirit and scope of the invention as defined in the claims. I will.
[Brief description of the drawings]
FIG. 1 is a perspective view of a non-closed magnet of the present invention.
FIG. 2 is a side view of the non-closed magnet of the present invention.
FIG. 3 is a front view of a non-closed magnet when positioned in a horizontal imaging position.
FIG. 4 is a front view of a non-closed magnet when positioned in a vertical imaging position.
FIG. 5 is a cross-sectional view of a first embodiment of a molded pole piece of the present invention.
FIG. 6 is a cross-sectional view of a second embodiment of the molded pole piece of the present invention.
FIG. 7 is a cross-sectional view of a third embodiment of a molded pole piece of the present invention.
[Explanation of symbols]
10 MR open magnet device 12 Magnet assembly 14 Superconducting coil assembly 16 Magnetic field enhancer 18 Support frame 20 Pedestal 24 Imaging volume 118, 218, 318 First cylindrical portion 120, 220, 320 Second cylindrical portion 122, 222,322 Frustum portion 224,324 Outer end face 230 Conical recess 332 Annular groove

Claims (1)

A pedestal, an aluminum C-shaped support frame rotatably attached to the pedestal, a first ferromagnetic magnetic field enhancer attached to one end of the C-shaped support frame, and a C-shaped support frame. A second ferromagnetic field enhancer attached to another end, including first and second superconducting coils;
The first magnetic field enhancer and the second magnetic field enhancer are spaced parallel to each other so as to form an imaging volume therebetween;
Further, each of the first and second magnetic field enhancers includes:
A first cylinder having a plurality of concentric annular grooves having different groove depths and having an end face opposite to the imaging volume and surrounding the first or second superconducting coil Part ,
A second cylindrical portion having an end surface facing the imaging volume having a diameter larger than that of the first cylindrical portion and provided with a recess ; and
A frustoconical portion connecting the first and second cylindrical portions ;
The first magnetic field enhancer and the first superconducting coil form a first magnet assembly, and the second magnetic field enhancer and the second superconducting coil are a second magnet assembly. An open-type magnetic resonance imaging magnet apparatus characterized by comprising:

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